A. Field of the Invention
The invention relates to electromagnetic acoustic transducers usable with cylindrical objects, and methods for determining resonant frequencies and physical properties of cylindrical objects using electromagnetic acoustic transducers.
B. Summary of the Invention
The invention is directed to an electromagnetic acoustic transducer (hereinafter "EMAT") adapted for use with cylindrical objects, and to methods for using an EMAT to determine the resonant frequencies and physical properties of a cylindrical object. "Cylindrical object" is used to denote a body having a cylindrical shape with an approximately circular cross section. The term cylindrical body includes solid cylindrical bodies and hollow cylindrical bodies such as pipes and tubes.
The EMAT of this invention includes a housing having a circular opening, a plurality of magnets mounted in the housing at evenly spaced intervals around the circular opening, and at least one wire coil mounted in the housing adjacent the circular opening and the polar ends of the plurality of magnets. Applying an electrical excitation signal to a wire coil of the EMAT will excite vibrations in a cylindrical object inserted into the housing of the EMAT. When the excitation signal is at a resonant frequency of the cylindrical object, the cylindrical object resonates, i.e., the forces applied to the cylindrical body by the EMAT constructively interfere with the natural vibrations of the cylindrical body, and large amplitude vibrations are produced. Once the cylindrical object is vibrating, the EMAT of this invention is also able to sense the amplitude and frequency of the vibrations in the cylindrical object.
The EMAT of this invention excites very specific types of vibrational motion in the cylindrical object inserted into the circular opening. Depending on the orientation of the wire coil relative to the magnets and the cylindrical object the EMAT can induce axial shear vibrations, torsional vibrations, radial vibrations or plain strain vibrations.
The EMAT of this invention is also useful in determining the frequencies at which a cylindrical object experiences resonant vibrations in each of the above identified types of vibrational motion. In addition, various physical properties of the cylindrical object, or the loading applied to the cylindrical object can be determined by:
(1) determining the amount of time it takes for resonant vibrations in the cylindrical body to decrease to a negligible value;
(2) comparing resonant frequencies of the cylindrical object to resonant frequencies of a standardized cylindrical object made from the same material and having approximately the same dimensions;
(3) determining how the resonant frequencies of several types of vibrational motion change as a load is applied to the cylindrical object; and
(4) determining how the amplitude of resonant vibrations change as the object is rotated within in the EMAT.
In particular, this invention provides a non-contact type EMAT usable with cylindrical objects.
This invention also provides an EMAT capable of exciting specific types of vibrational motion in the cylindrical object.
This invention further provides an EMAT capable of exciting axial shear vibrations, torsional vibrations, radial vibrations, or plain strain vibrations in a cylindrical object.
This invention provides a method for using an EMAT to determine the resonant frequencies of a cylindrical object.
This invention also provides a method for using an EMAT to determine the resonant frequency or a cylindrical object for axial shear vibrations, torsional vibrations, radial vibrations, or plain strain mode vibrations.
This invention further provides methods for using an EMAT to measuring physical properties of cylindrical objects.
This invention further provides a method for using an EMAT to determine the radial depth from the exterior surface of a cylindrical object at which physical properties of the material of the cylindrical object undergo a change.
This invention also provides a method for using an EMAT to determine the magnitude of a load placed on a cylindrical object.
This invention further provides a method for using an EMAT to determine the texture or grain orientation of the material forming a cylindrical object.
These and other objects and advantages of the present invention will become apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.